The invention relates to a method for reducing the supply of air from the atmosphere into the expansion vessel of high-voltage plants filled with insulating liquid. Furthermore, the invention relates to an apparatus for carrying out the method that differs with the new commissioning of transformers from that of transformers with already started thermal aging.
High-voltage plants, e.g. transformers, are filled with insulating liquids such as mineral oils for cooling. Load changes, variations in the performance of the cooling plants, and ambient temperatures lead to distinct temperature changes, and thus to changes of the volume of oil. The oil is received by expansion vessels above the transformer tank. In these vessels, there is a direct contact of the oil level with the atmospheric air. Pressure compensation is carried out via a conduit which at its end is sealed with an air dehumidifier and an oil cone. Additionally, air is drawn from the atmosphere at the beginning of thermal aging, when oxygen is consumed in the transformer, and also absorbed by degassed insulating liquids during new installations and repairs. Although the sealing system to the atmosphere has been successful in Europe, developments lead towards air-sealing systems that exclude the oxygen and also bypass air dehumidifying. A direct correlation can be seen from oxygen to the lifetime of the insulating system. There is both a lack of criteria for this and of reliable methods of analysis to monitoring thereof.
The known technical solutions substitute the direct air contact by use of separating diaphragms or enclose nitrogen or vacuum in the expansion vessel. These solutions have the following disadvantages:                high costs; especially with retrofittings;        retrofitting during the de-energized state;        lack of criteria for the efficiency;        due to technical limits the intended complete elimination of oxygen cannot be put into action.        
Since the complex role of oxygen has not yet been clarified, so far only the lowering of oxygen content has been attained.
There are known techniques which carry out a separation of the active part in the oil itself. DE 102005054812 A1 discloses a tubular formed hollow body situated in parallel to a tank and hydraulically connected to the tank. A floating disposed sealing piston is guided therein which is loaded with an insulating liquid of a defined electrical stability of the filling of insulating oil in the tank, on the one side, and with an insulating oil being under atmospheric pressure and having any electrical stability, on the other side, wherein the insulating oil serving as blocking liquid is located in an compensation container arranged above the hollow body.
DE 10035947 B4 discloses a device for reducing the contamination of liquids caused by exposure to air and water. The device is comprised of a main reservoir in which a heat source is located that in its lower area is connected to the expansion container through a pipe leading to the ambient atmosphere. Between the pure and warm liquid, a stable layer of the heat stratification forms spontaneously under the heat source at the boundary layer to the cold, potentially contaminated liquid located beneath, which is disposed in the lower area of the main reservoir, the connecting pipe, and the expansion container.
The above mentioned disadvantages also apply to these techniques.